Leading the way along the Anhinga Trail, an elevated boardwalk that twists and turns for half a mile through the Florida Everglades National Park, Don DeAngelis '66 stops suddenly and points out a full-grown alligator sunning itself 20 yards ahead. When the gator's jaws groan open in what appears to be a monster-sized yawn, he chuckles with delight.

"I think it's pretty clear that the alligator population of the Everglades is doing okay," says DeAngelis, a theoretical ecologist with the U.S. Geological Survey (USGS), who has spent two decades building complex mathematical models to evaluate the viability of south Florida's threatened animal species. "In recent years, I've counted up to 50 alligators during a single walk along this trail."

That's a welcome change. As two more gators emerge from a murky creek, DeAngelis recalls that the alligators of the Everglades were on the federal endangered-species list from the mid-1960s until the mid-1980s. Today their survival is no longer at issue.

"Unfortunately, that can't be said about some other Everglades species, such as the Florida panther and the wood stork," he says. "Right now, we've got at least a dozen species that are very much in peril because of continuing pressure from agriculture, development, and sometimes ineffective water management policies."

DeAngelis's computer-generated simulation models of animal habitats are playing a critical role in the effort to save Florida's endangered species through the Comprehensive Everglades Restoration Plan (CERP), a 30-year, $9.5 billion federal program. "The state and the federal government are making progress in restoring some parts of the Everglades since CERP began in 2000," he says. "But I'm still concerned about the long-term future of this ecosystem. We've made important gains, but I'd say the jury is still out on how healthy the Everglades will be in 50 years."

As DeAngelis treks along the trail through a vast freshwater region of shallow saw-grass marshlands and areas of deeper water known as sloughs, he finds himself eyeball to eyeball with at least a dozen alligators (he guesses that perhaps 35,000 now roam the 2,000 square miles of Everglades National Park's wetlands, which stretch from Miami south to Florida Bay), as well as other creatures whose habitats he studies. Finally, he leads the way to a swampy tract covered with three inches of "sheet flow," the barely moving surface water that typifies the great saw-grass marsh.

"The deeper areas of this sheet flow provide a habitat for apple snails, which are the only food source for the snail kite," he says, referring to a threatened bird species. Drought, faulty water management, and surging development have reduced water levels, depriving them of their food supply. "Right now the total Everglades population of snail kites is way down, and that's disturbing," he says.

In fact, DeAngelis notes, the hydrology of the entire Everglades has changed significantly during the past four or five decades. Nearby sugarcane farming and other forms of agriculture have encroached on the area as the population of south Florida has soared from about 1.3 million to nearly 4.5 million people. "There's been relentless pressure from population growth, which has led to building numerous dikes and canals that can interfere with normal water flow," he says. As a result, the 5,000-year-old Everglades have shrunk from an original 8,000 square miles to slightly less than half that today.

Since the early 1990s, DeAngelis has coördinated the building of habitat and population models that the environmental agencies involved in CERP can use to evaluate how various water management scenarios might affect threatened species such as the snail kite. An international team of scientists and students works on the project, known as Across Trophic Level System Simulation (ATLSS).